A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma (LPP) sources, discharge-produced plasma (DPP) sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation source apparatus for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation source apparatus is typically termed a laser produced plasma (LPP) source.
As a by-product of the plasma generation, debris of the fuel material in the form of vapor, dust or droplets is ejected in many directions, as well as the wanted EUV radiation. The radiation source apparatus typically includes many structures and subsystems to manage this material, which otherwise would quickly contaminate optical surfaces and degrade performance. One such measure is to provide a receiving structure that surrounds the beam to intercept and trap the fuel debris. The structure may be heated to a temperature at which the debris will melt and run into drainage channels so that it can be removed from the environment, whether occasionally or continuously during operation. An example of such an apparatus is disclosed in published patent application US 2008179548. To liquefy the debris in the case of tin as a fuel, the temperature of the receiving structure surface may be approximately 300° C.
To maximize the intercepting surface and discourage rebounding of debris back into the environment, the receiving structure typically has a convoluted surface, for example covered in fins or vanes or other local structural elements. The heating of all these elements to the correct temperature brings many challenges. The patent application US 2008179548 mentions resistive heating, or alternatively fluid heating using water or liquid gallium. Because of the large number local structural elements requiring their own heating, the heating system becomes very complex. The prevailing method example of resistive heating requires a thermally conductive connection from each heating element into the receiving structure. Materials used for this element and connection must be EUV- and vacuum-compatible, limiting the choice of materials. Inefficiency in the transfer of heat to the structure implies that the elements themselves reach a much higher temperature. This brings further challenges for the selection of materials and reliability.